Many technologically important processes have to be carried out in a high vacuum. Among such processes are semiconductor manufacturing processes such as molecular beam epitaxy (MBE). Frequently the quality of the resulting product strongly depends on the residual pressure in the vacuum processing chamber. This is, for instance, the case in the field of III-V semiconductors, where carrier mobility in at least some materials has been found to strongly depend on the vacuum-chamber pressure during MBE growth. Thus there exist strong incentives for carrying out such processes in the best vacuum (i.e., under the lowest pressure) that can be attained economically.
Any vacuum system comprises an evacuable chamber and means for removing gas from the chamber, typically one or more vacuum pumps. The pump is connected to the chamber in a manner that results in relatively low impedance for gas transport between the chamber and the pump.
Many different types of vacuum pumps are known. Among pumps capable of producing high vacuum are cryopumps, ion sputter pumps, getter pumps, and turbomolecular pumps. Of these, cryopumps have perhaps the most desirable combination of properties, among which are cleanliness and high pumping speed per invested dollar. It is probably for this reason that cryopumps are now increasingly being used in UHV apparatus.
A cryopump works by adsorbing gaseous molecules onto surfaces maintained at cryogenic temperatures by, exemplarily, a closed-cycle, gaseous helium refrigerator. The pumping surfaces typically are enclosed within a cylindrical stainless-steel canister which is flanged on the open end for joining to a pumping port of a vacuum chamber. The operation of cryopumps is described, for example, in J. F. O'Hanlon, A User's Guide to Vacuum Technology, John Wiley and Sons, New York, 1980.
In any otherwise leak-free vacuum system, gas enters the open volume of the vacuum chamber by surface desorption from the chamber walls or permeation through the walls. The ultimate vacuum attainable may be determined by the competition between the pumps and such outgassing or permeation. The room-temperature outgas rate can be drastically reduced by preliminarily baking the walls of the vacuum system, typically at a temperature between 200.degree. C. and 300.degree. C., while pumping to maintain the vacuum within the system.
The far-UHV (Ultra-High Vacuum) region comprises the pressure range below approximately 10.sup.-9 Pa. (1 Torr is approximately equal to 133 Pa.) Base pressures in this range are important, exemplarily, in the growth of advanced materials by molecular beam epitaxy. During MBE growth, residual gases in the vacuum chamber may be a significant source of contamination of the product. For that reason, far-UHV conditions are needed in order to achieve desired high levels of material purity and structural perfection.
In order to reach the far-UHV region, a vacuum system must be bakeable to the maximum extent possible. In other words, the greatest possible fraction of the inner surface of the system must be desorbed by heating to substantially eliminate outgassing during subsequent, post-bake operation, with the exception of surfaces that are maintained at cryogenic temperatures during operation.
The following example demonstrates the importance of maximum bakeability. Far-UHV systems are preferentially made of stainless steel. The 22.degree. C. outgas rate of unbaked stainless steel drops from 10.sup.-7 Pa-m/s to 10.sup.-11 Pa-m/s after a 100-hour bake in UHV at 200.degree. C. Thus if only 1% of the 22.degree. C. stainless-steel surface of a UHV system is left unbaked, it will surely become the dominant outgas source of the entire system, and limit the attainable vacuum. As already noted, however, there is no need to bake the cryogenic surfaces within the pump, because their outgas rates during operation will be negligible even at 10.sup.-12 Pa.
Closed-cycle, gaseous helium-cooled cryopumps potentially offer distinct advantages as far-UHV pumps: they are oil-free, can be made to pump all gases, do not suffer from unintentional regurgitation, and provide as much as ten times the pumping speed per dollar as other types of UHV pumps. However, commercially available cryopumps cannot be UHV baked. The thermal load that would be seen by the cryopump if its stainless-steel walls were baked would have to be absorbed by the first refrigeration stage shield (the "primary pump stage") nominally held at 77.degree. K. This thermal load during baking typically can not be handled by the cooling system of prior art pumps, as is demonstrated by the following example, which is based on a typical commercially available cryopump. The exemplary cryopump has a cooling capacity of 50 watts at the first stage using the largest optional compressor provided by the manufacturer. This is less than 25% of the thermal power delivered to the pump by the walls of the cryopump when it is baked at 200.degree. C.
Despite the problem of outgassing from the canister, non-bakeable cryopumps have been installed on many bakeable vacuum systems. One approach to the outgas problem has been to baffle the pumping path between the cryopump and the vacuum chamber in order to reduce the thermal load to the cryopump during the baking of the vacuum chamber. This measure is not very satisfactory because it reduces the pumping speed, and it does not provide for outgassing of the baffle or pump walls.
One way to make the cryopump bakeable is to provide for the temporary removal of heat-sensitive mechanical parts. For example, one published report (see M. Michaud and L. Sanche, "Characteristics of a Bakeable Ion-Cryopumped UHV System," J. Vac. Sci. Technol., 17 (1), January/February 1980, pages 274-276) describes a bakeable UHV system comprising a modified cryopump and an auxiliary ion pump, both communicable with the UHV chamber. In order to make the cryopump bakeable, heat-resistant materials were substituted for heat-sensitive materials within the pump. However, before baking the pump, it was necessary to remove mechanical components of the pump refrigeration system. During bakeout, the cryopump was not operated, and vacuum pumping was provided only by the auxiliary ion pump. A steady-state vacuum-chamber pressure of 1.3.times.10.sup.-8 Pa was achieved by this means, and the pressure could for a limited time be brought down to 8.0.times.10.sup.-9 Pa.
Removal of heat-sensitive mechanical components is similarly taught in U.S. Pat. No. 4,514,204. The cryopump disclosed in this reference comprises a cryogenic refrigerator whose displacer end can be removed from the cryopump housing without loss of vacuum. During bakeout, the refrigeration means that were removed can be used through a non-heated port to continue pumping, thus obviating the need for a separate ion pump to maintain the vacuum during bakeout.
Although prior-art bakeable cryopumps with removeable refrigerators offer advantages over non-bakeable cryopumps, they are inconvenient to operate and during bakeout they require costly ancillary pumping equipment which may itself be disadvantageous. For example, ion pumps suffer from the re-emission of previously pumped gases, and the use of a non-heated auxiliary port for continued cryopumping partially defeats the purpose of having a bakeable pump. Thus there is a need for a bakeable vacuum system, comprising a bakeable cryopump, in which the cryopump can be operated during bakeout. This application discloses such a system.